In producing developed fields, it is necessary, prior to transporting well fluids from an offshore location to storage or refining facilities, to separate the gas, oil, water and other components of produced well fluids. Also, it is often necessary to control and meter the production of wells. Initially, this was done using conventional wellhead equipment from a fixed platform. Later, as subsea production technology advanced, the wellhead equipment was installed on the ocean floor and risers installed between the wellhead and the surface production platform.
As offshore oil field operations moved into deeper waters, i.e. 400 to 1,000 feet depths, semi-submersible vessels supplanted fixed structures as the production platform. Also, as a part of economic justification for the additional expense attendant operations in these deeper waters, the fields developed were larger, requiring that number of individual wells be drilled in different parts of the field. Thus, the arrangement for producing deep water fields became one wherein a number of satellite wells, each with its own subsea wellhead equipment, are interconnected with flow lines to a central manifold and riser. The manifold and riser serve to conduct crude oil to processing tanks on the semi-submersible and to a swivel buoy mooring for loading into a tanker. The equipment for implementing the satellite system has heretofore consisted of a number of satellite production trees and a production manifold and riser system.
In a satellite production system, each well is independently established by drilling a borehole and then landing the well control equipment thereon. In establishing a satellite well, a temporary guide base is placed on the ocean floor to compensate for possible sloping of the floor and to provide guidance for drilling the pilot hole. Wire guidelines are connected to the guide base at the surface prior to lowering it to the ocean floor. These wire guidelines are used in guiding re-entry to the well during all subsequent steps in establishing a producing well. Such wire line guides may be attached to the guide base in several different ways, for example, spears adapted for shear release from a receptacle or adapted release from spring-loaded segments for non-shear retention.
After the drill string is retrieved, an outer conductor is entered into the well bore and held in place within radial latches built into the center ring of a permanent guide base and which attach to a suspension joint on the outer conductor. The permanent guide base also includes removable corner posts, each of which has a longitudinal slot to permit insertion therein of the four wire guidelines attached to the temporary guide base. The outer conductor is jetted into place, and a guide assembly with the drill string held therein is lowered to re-enter the bit into the well bore.
Prior to the beginning of drilling of a hole for an inner conductor string, a lower riser package assembly is lowered and latched to the outer conductor. Drilling of a smaller casing hole is then performed through the riser, with the riser providing directional control to bleed off any shallow pressured sands penetrated. The lower riser assembly is retrieved upon completion of the casing hole, and the inner conductor string is run into the borehole and set.
The subsea wellhead equipment, or Christmas tree, is then installed. The equipment is made up of several components, the lowermost component being a conductor which attaches to the inner conductor and is capable of being latched or unlatched remotely by means of a hydraulic control system. Atop the connector is a block valve assembly which includes valves for providing outlet flow paths for the crude. The wellhead equipment is mounted on a guide frame adapted for the wire line guidance onto the permanent guide base.
The central manifold and riser of a satellite production system is interconnected with the individual wells by flow lines independently laid. Typically, dual flow lines between each well and the central manifold are used. The attachment of flow lines to the wellhead equipment is by diver manipulated, hydraulically actuated connectors. Occasionally, flow line connection is made utilizing a fixed spool made up on the site using a diver fabricated jig.
The central manifold has two main sections. The lower section is called the permanent base; it provides the attachment points for the flow lines. Mounted on the permanent base section are stab receptacles for landing the second section, the manifold. The upper manifold section contains all of the hydraulic connectors for securing the manifold section to the permanent base and for interconnecting with the production riser. The manifold section also includes several valves for directing the flow of crude oil to various riser tubes.
The riser consists of a central tube and several nonintegral, externally guided tubes, each having a smaller diameter than the central tube. The smaller tubes carry high pressure crude from the manifold to the deck of the semi-submersible; and the central tube conducts processed crude at low pressure down to a swivel buoy moored loading facility.
Although the establishment of a producing subsea in the aforementioned manner proceeds quite well in waters having depths up to about 1,000 feet, when it is necessary to establish a subsea production system for large fields in substantially deeper waters, i.e. 3,000 feet and deeper, the utilization of wire line guidance and diver attachment of flow line connections is not feasible. In waters of such substantial depth, it becomes necessary to employ a dynamic guidance system utilizing sonar and T.V. monitoring of re-entry to the well, landing of wellhead equipment, and connection of flow lines.
As can readily be appreciated, the making of flow line connections by remote control from the surface is very tedious and time consuming. Also, even with visual contact provided by T.V. monitoring, the landing of wellhead equipment becomes a much more complicated endeavor.